Conventional tungsten lamps or fluorescent lamps of white light illumination devices have been gradually replaced by commercially available light emitting diodes (herein after LEDs). Advantages of LEDs are as follows: (1) the small size of LEDs make them suitable to be illumination devices of array packages and collocated with different colors, if necessary; (2) LEDs have relatively long operating life spans of more than 10,000 hours, which is 50 times that of conventional tungsten lamps; (3) LEDs have shock resistance and are durable due to transparent resin applied as a packaging resin; (4) LEDs are environmentally friendly as the interior structures are free of mercury; (5) LEDs consume ⅓ to ⅕ less power than that of the conventional tungsten lamp.
Conventional white light illumination devices achieve a white emission by yellow YAG phosphor excited by a blue LED (with a wavelength of 450±20 nm). The described two-wavelength (blue and yellow) white LED has many illumination limitations due to the high color temperatures and uneven colors. Therefore, color quality is less than desired. Also, color control of the white light is difficult due to the blue light wavelength from the LED chip changing along with different temperatures thereof. In addition, the two-wavelength white light lacks red light, thereby reducing color rendering thereof.
Therefore, it is necessary to develop a novel red phosphor suitable for use in a white light illumination device to increase the color rendering index thereof.